Learning Objectives
Master the pathophysiology and clinical presentation of Twin-Twin Transfusion Syndrome (TTTS). Understand the role of arteriovenous anastomoses and distinguish between the donor and recipient twin phenotypes for the USMLE Step 1.
1. Pathophysiology: The Vascular Shunt
TTTS occurs exclusively in monochorionic (shared placenta) twin gestations. It is driven by unbalanced vascular connections within the placenta.
| Component | Mechanism |
|---|---|
| Anastomoses | Unbalanced arteriovenous (AV) anastomoses. |
| Blood Flow | Net blood flow shifts from one twin (Donor) to the other (Recipient). |
| Placentation | Can ONLY occur in monochorionic twins (Mo-Di or Mo-Mo). |
2. The Donor Twin (“Stuck Twin”)
The donor twin loses blood volume to its sibling, leading to significant growth restriction and fluid issues.
| Clinical Feature | Pathophysiology |
|---|---|
| Hypovolemia | Decreased blood volume and anemia. |
| Oligohydramnios | Low urine output due to poor renal perfusion. |
| Appearance | “Stuck twin” (looks shrink-wrapped against the uterine wall). |
3. The Recipient Twin
The recipient twin receives an excess of blood, which places a massive strain on its cardiovascular system.
| Clinical Feature | Pathophysiology |
|---|---|
| Hypervolemia | Excess blood volume and polycythemia. |
| Polyhydramnios | Excessive fetal urination due to high volume. |
| Heart Failure | Volume overload can lead to hydrops fetalis. |
Activity
High-Yield Clinical Pearls:
- The Vascular Culprit: If a question asks for the underlying cause, the answer is AV anastomoses.
- Hydrops Risk: The Recipient is actually at a higher risk for heart failure and hydrops fetalis due to the extreme volume overload.
- Diagnosis: Look for a significant size discrepancy and fluid imbalance (Oligo-Poly sequence) on ultrasound in a monochorionic pregnancy.